Transmission system for telephone communications or data transfer

Abstract

The invention concerns a transmission system for telephone communications comprising at least two telephone sets, each incorporating a microphone and a receiver and/or loudspeaker, and a telephone line connecting the telephone sets by at least two conductors in order to form a current loop. Connected to the current loop is an alternating-current generator and current-steering switch elements arranged to steer the current to be routed during the positive half-cycles generated by the generator via the microphone circuit of the first set and the receiver and/or loudspeaker circuit of the second set as well as to steer the current during the negative half-cycles via the receiver and/or loudspeaker circuit of the first set and the microphone circuit of the second set, whereby voice transmission takes place by sampling the microphone signals of the interconnected telephone sets and transmitting the alternate signal samples superimposed on the generator signal so that the microphone signal of the first set is superimposed on the positive half-cycles of the generator signal while the microphone signal of the second set is superimposed on the negative half-cycles of the generator signal. The invention is also applicable to data transmission, wherein the telephone sets are replaced by modems of a simple construction.

What is claimed is: 
     
       1. A transmission system for telephone communications comprising at least two telephone sets, each incorporating microphone (9) and a receiver and/or loudspeaker (10), a telephone line connecting the telephone sets by two conductors in order to form a current loop, an alternating-current signal generator (2) and current-steering switch elements (3, 4; 6, 7 or 11) connected to the telephone sets and the conductors, arranged to steer the current to be routed during the positive half-cycles generated by the generator via the microphone circuit (9) of the first set and the receiver and/or loudspeaker circuit (10) of the second set as well as to steer the current during the negative half-cycles via the receiver and/or loudspeaker circuit (10) of the first set and the microphone circuit (9) of the second set, whereby voice transmission takes place by sampling the microphone signals of the interconnected telephone sets and transmitting the alternate signal samples superimposed on the generator signal so that the microphone signal of the first set is superimposed on the positive half-cycles of the generator signal while the microphone signal of the second set is superimposed on the negative half-cycles of the generator signal, characterized in that said current-steering switch elements are connected between a current loop comprising but two conductors, and a microphone circuit (9) and a receiver and/or loudspeaker circuit (10) at the telephone sets so as to prevent bidirectional current paths between the microphone circuit and the receiver and/or loudspeaker circuit within a single telephone set, while allowing only a unidirectional current path of the current loop through said circuits. 
     
     
       2. A system as claimed in claim 1 characterized in that the alternating-current generator (2), whose frequency is used as the sampling frequency, is connected to a current-loop, which includes the current-switching elements (3, 4; 11) of the second telephone set. 
     
     
       3. A system as claimed in claim 1, characterized in that the alternating-current generator is a square-wave generator. 
     
     
       4. A system as claimed in claim 1, in which system the current loop connecting the telephone sets is routed via a telephone exchange, characterized in that the alternating-current generator and a part of the current-switching elements are placed at the exchange equipment. 
     
     
       5. A system as claimed in claim 1, characterized in that the current-switching elements are diodes (11). 
     
     
       6. A system as claimed in claim 1, characterized in that the current-switching elements incorporate a bandpass filter (5). 
     
     
       7. A system as claimed in claim 1, characterized in that the current-switching elements incorporate switches (3, 4; 6, 7), placed separately at both ends of both current-loop conductors and configured so that at any time, two switches (3 and 4 or 6 and 7, respectively) belonging to a single telephone set are synchronized to operate pair-wise in such a manner that while one pair (6; 3) forms a bypass route for the telephone circuit (10) current, the other pair (7; 4) connects the telephone circuit (9) associated with the switch pair onto the current loop. 
     
     
       8. A telephone set compatible with the system claimed in claim 1, characterized in that the telephone set terminals to be attached to a paired telephone line are electrically connected to a microphone (9) and a receiver and/or loudspeaker (10, 10') by means of current-switching elements (11), which are configured so that current path via the microphone circuit (9) in the current loop of the line is unidirectional bypassing the receiver and/or loudspeaker circuit (10'), while the current path via the receiver and/or loudspeaker circuit (10') is counter-directionally unidirectional bypassing the microphone circuit (9). 
     
     
       9. A transmission system comprising at least two communications sets which each incorporate means for receiving and transmitting incoming and outgoing communications signals respectively connected by a paired line current loop, an alternating-current signal generator connected to generate current in the current loop, and current-switching elements connected to each of the communications sets which implement communications signal transfer by means of samples taken alternatingly from the communications signals of the interconnected communications sets, said communications signal samples being transmitted superimposed on the generator signal so that the outgoing communications signal of the second set is superimposed on the negative half-cycles of the generator signal and the outgoing communications signal of the second set is superimposed on the negative half-cycles of the generator signal, characterized in that said current-switching elements are arranged to steer the current during the positive half-cycles of the generator signal to be rout ed via the transmitting means of the first set and the receiving means of the second set, bypassing the receiving means of the first set and the transmitting means of the second set, and to steer the current during the negative half-cycles of the generator signal to be routed via the receiving means of the first set and the transmitting means of the second set, while correspondingly bypassing the transmitting means of the first set and the receiving means of the second set. 
     
     
       10. A system as claimed in claim 9 wherein the alternating-current signal generator, whose frequency is used as the sampling frequency, is connected to the current loop which includes the current-steering elements of the second communications set. 
     
     
       11. A system as claimed in claim 9 wherein the alternating-current signal generator is a square-wave generator. 
     
     
       12. A system as claimed in claim 9 wherein the current loop connecting the communications sets is routed via a telephone exchange, characterized in that the alternating-current signal generator and a portion of the current-steering switch elements are located at the telephone exchange. 
     
     
       13. A system as claimed in claim 9 wherein the current-steering switch elements are diodes. 
     
     
       14. A system as claimed in claim 9 wherein the current-steering switch elements incorporate switches placed separately at both ends of both current loop lines and configured so that at any time two of the switches belonging to each communications set are synchronized to operate together in such a manner that while one switch forms a bypass route for the current for the receiving means, the other switch connects the transmitting means with the current loop. 
     
     
       15. A system as claimed in claim 9 wherein the communications sets are telephone sets attached to a paired telephone line as the current loop, the telephone sets each having a microphone as the transmitting means and a receiver and/or loudspeaker as the receiving means connected to the current-steering switch elements and configured so that a current path via the microphone in the current loop of the telephone line is unidirectional bypassing the receiver and/or loudspeaker, while the current path via the receiver and/or loudspeaker is counter-directionally unidirectional bypassing the microphone. 
     
     
       16. A system as claimed in claim 9 wherein the communications sets are modems attached to the paired line as the current loop, the modems each having an outgoing interface as the transmitting means and an incoming interface as the receiving means connected to the current-steering switch elements configured so that a current path via the incoming interface in the current loop of the paired line is unidirectional bypassing the outgoing interface, while the current path via the outgoing interface is counter-directionally unidirectional bypassing the incoming interface.